The overall goal of this research is to advance the understanding of flexibility in motor skill acquisition - how infants, children, and adults learn to select appropriate movements and modify them as necessary in accordance with the current constraints on action. Flexibility is imperative for adaptive motor decisions because the constraints on action (e.g., environment, bodies/skills, and penalties for error) are continually changing. Empirical work indicates that everyday experience with balance and locomotion provides infants the wherewithal to flexibly adapt motor decisions to novel changes in their bodies, skills, and environment. Learning, however, is specific to each perception-action system in postural development (sitting, crawling, and walking). Specific Aim #1 is to test the flexibility and specificity of infants'motor decisions. The proposed experiments use an age-matched design to test flexibility within experienced perception-action systems under conditions of altered body dimensions/skills and penalties for error and to test specificity between experienced vs. newly acquired perception-action systems. Specific Aim #2 is to describe the time course of flexibility. The proposed studies use a microgenetic design to test experience-related changes in infants'ability to cope with novel constraints on action and a cross-sectional design to test age-related changes in flexibility from infancy to old age. Specific Aim #3 is to assess flexibility in "specialized" perception-action systems - skills acquired after infancy by only a subset of children due to specific opportunities and demands. Proposed studies use a "monkey bar" paradigm to test children's motor decisions under conditions of altered body constraints and penalties for error. Across all experiments, a psychophysical method will be used to quantify the correspondence between motor decisions and actual possibilities for action. As the penalties for errors in the proposed studies are falling or entrapment, findings will provide insight into these two leading causes of accidental injury. Falling from monkey bars is the leading cause of playground injury in middle childhood. In addition, the proposed methods, conceptual framework, and findings can provide the basis for assessing flexibility in children with motor impairments and for tracking improvements in flexibility with therapy.